Circuit for generating from low voltage edges higher voltage pulses having precise amplitudes and durations

ABSTRACT

A low voltage source of input edges is AC coupled to positive and negative edge differentiators. Each differentiator briefly drives ON a corresponding normally OFF switch, the pair of which switches serves as an impulse driver and that may be clamped to symmetrical opposing reference voltages. The bi-stable switch driver responds to a particular polarity impulse driver voltage by applying thereto through a load resistor a percentage of a power supply voltage of opposite polarity. The impulse driver is also coupled to a complementary pair of MOSFET switches that are in series between precise reference voltages of opposing polarities, and whose common junction may be connected to a current determining resistance. The output of the MOSFET switches may be assisted by an auxiliary bi-stable output driver that reduces the amount of load current drawn from those MOSFETS.

BACKGROUND OF THE INVENTION

The use of pulses having precisely defined widths is common in manytypes of electronic circuits. Such pulses can be created with digitallogic. It often happens that such a pulse is required to define morethan just a width in time; for example, when it is applied to anintegrator that is also responsive to the amplitude of that pulse. Acommon instance of this is when a pulse of precision width and voltagedrives an integrator input resistor. Such precision current pulses musthave not only a precise duration, but also a precise amplitude, sincethe total amount of charge applied to the integrator is (for an ideallyshaped pulse) a product of the two. Precise current pulses are oftenused in conjunction with integrators in such applications as DACs(Digital to Analog Converters), which in turn may be used in theprecision ADCs (Analog to Digital Converters) that are found inlaboratory quality digital multi-meters.

Two trends in current practices have introduced a difficulty in thedesign of a precision pulsed current source. The first is the trend toplace ever more circuitry onto a single IC (Integrated Circuit). Whilethis has many advantages in some respects, it does limit what mix ofsemiconductor processes is available, and places a premium of keepingpower dissipation to a minimum. A second trend in response to the firstis that the industry has developed low voltage high speed logic familiesthat operate on 5V or less. It is possible to generate high speedprecisely placed pulse edges with these low voltage logic families, butwhen converted to current pulses the low voltages involved give rise toundesirable reductions in signal to noise ratios. Very often, thedesigner would prefer to have a much higher voltage to work with, but iscompelled to either use the low voltage associated with the newer logicfamily and suffer the loss in precision, or use an older (and perhapsless available) process with a higher voltage. A third option is to usea modern low voltage high speed logic family for pulse-edge generation,and then apply that to a switching mechanism to shift its level to asuitably high voltage obtained from a stable reference voltage. A numberof challenges arise when this latter approach is used in a system thatis expected to be extremely accurate, and just how to do it is asignificant problem in its own right.

SUMMARY OF THE INVENTION

A low voltage source of input edges for a current pulse of precisionamplitude and duration is applied to an input buffer to provide a lowimpedance source whose output is AC coupled to a positive edgedifferentiator and also to a negative edge differentiator. Eachdifferentiator briefly drives ON a corresponding normally OFF switch,the pair of which switches are in series between first power supplies ofopposite polarity, and whose junction serves as an impulse driver andthat may be clamped to symmetrical opposing reference voltages less thanthe voltages of the first power supplies. The clamped impulse driver mayalso be coupled to a bi-stable switch driver that responds to a positiveimpulse driver voltage by applying thereto through a coupling resistor apercentage of the negative first power supply voltage. The bi-stableswitch driver responds to a negative impulse driver voltage by applyingthereto through the coupling resistor a percentage of the positive firstpower supply voltage. The bi-stable switch driver changes state eachtime input edges of alternating polarity are applied to the inputbuffer. The impulse driver is also coupled to the gates of acomplementary pair of MOSFET switches that are in series between precisereference voltages of opposing polarities, and whose common junction maybe connected to a current determining resistance whose other end isconnected to a using circuit, such as an integrator. The output of theMOSFET switches may be assisted by an auxiliary bi-stable output driverthat reduces the amount of load current drawn from those MOSFETs, andthereby reduces errors arising from thermally induced variations intheir ON resistance. The precise reference voltages may be the same asthe symmetrical opposing references.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of a voltage measurementarchitecture that provides an occasion for practicing the invention;

FIG. 2 is a simplified schematic portion of the DAC mechanism in FIG. 1,wherein current pulses are generated from the low voltage edges of apulse width modulated signal; and

FIG. 3 is a schematic portion of the DAC mechanism in FIG. 1, andincludes further improvements for greater accuracy and linearity.

DESCRIPTION OF A PREFERRED EMBODIMENT

Refer now to FIG. 1, wherein is shown a simplified block diagramrepresentation 1 of a voltage measurement technique known as the ΔΣarchitecture. We shall describe it briefly, as the subject matter we areinterested in can be put to use in the ΔΣ architecture. The ΔΣarchitecture itself may be used, for example, in digital multi-metersranging from the mid-range of performance to equipment at the very topof the line (e.g., a multi-meter such as the Agilent 3458A).

A voltage V_(IN) 2 to be measured is applied to the + input of a SUMMINGJUNCTION 3, whose output is coupled to an INTEGRATOR 4. When, in thecourse of events, the measurement is concluded, a feed-back balancingvoltage (9) will be developed whose average value is equal to minusV_(IN). This is applied to the other input of the SUMMING JUNCTION 3,such that if this condition is maintained for a suitable period of time,the output of the SUMMING JUNCTION will be zero, and the output of theINTEGRATOR will effectively cease changing. The output of the INTEGRATORis applied to an ADC, and is a digital representation of V_(IN) (again,assuming that balance has been achieved). That digital representation isapplied to an output display mechanism (or perhaps put to some otheruse), and it is also applied to a DAC from which issues the feed backbalancing voltage 9. This has been a very simplified look at the ΔΣarchitecture, and we have ignored many topics important to an actualfull scale implementation, such as input amplifiers and attenuators.Nevertheless, it will serve out purpose here.

Our interest in FIG. 1 is in what goes on inside the DAC 8. It receivesa collection of digital values, probably of many bits representing, say,four to eight decimal digits. It needs to turn that back into an analogvoltage within some range, say, −7V . . . +7V. Let us assume that, as isoften done, the binary representation from the ADC 6 has been used tocreate pulses whose widths are precisely defined by low voltage edges ofperhaps, three to five volts. These pulse width modulated input signalsare too small to create average values up to seven volts that willcancel the applied input at the SUMMING JUNCTION. If we used the smalleramplitude pulse width modulated signals (say, 3-5V) directly, we wouldhave to scale V_(IN) down in amplitude, and would incur an increasedsignal to noise ratio during operation. Thus, we are compelled to“stretch” the low voltage pulse width modulated signals vertically (involtage) in order to proceed. How to do that is the subject matter ofFIGS. 2 and 3.

Refer now to FIG. 2, wherein is shown a simplified schematic diagram 10of a basic circuit for creating precision pulses from the edges oflow-voltage pulse width modulated signals 11. The low voltage edges 11are applied to a DIFFERENTIATING IMPULSE DRIVER circuit that has twoportions, one for each polarity transition in the low voltage edges 11.Buffer 12, capacitor 14 and transistor 16 respond to negativetransitions, while buffer 13, capacitor 15 and transistor 17 respond topositive transitions. Between transitions the transistors 16 and 17 areboth normally OFF. Consider a positive transition in the low voltageedges. Both capacitors will differentiate it, but only transistor 17will respond, by turning ON very briefly, producing a negative impulsederived from the negative power supply 20. The time constants are chosenso that the ON time is in the range of 2% to 5% of the maximum, ortotal, pulse width period. In a similar fashion, a negative transitionwill briefly turn transistor 16 ON, producing a positive impulse derivedfrom the positive power supply 19.

An advantage of the arrangement shown for transistors 16 and 17 is theyare mostly OFF, which is desirable for reduced power consumption andheating related behavior changes. In addition, it will be appreciatedthat their ON time is a function of just time constants, and NOT afunction of pulse width. If their ON times were a function of inputpulse width, then an avenue for undesirable non-linear operation isopened.

When transistor 17 turns ON, a brief negative spike, or voltage impulse,appears at node 18 and is applied to, and charges, capacitor 21. Thisnegative voltage will turn MOSFET switch 22 ON, while turning MOSFETswitch 23 OFF. Thus, a positive transition in the low voltage edges 11will result in the positive power supply voltage 24 being the source foran output pulse on output line 26 that is applied to resistor 27 (tomake it a current pulse for an integrator that is not shown). Thepositive voltage at output 26 will remain until a subsequent negativetransition in the low voltage edges 11 produces a positive impulse fromtransistor 16 that discharges capacitor 21 from its previous negativevalue and charges it to the new positive one. That causes MOSFET switch23 to turn ON, and MOSFET switch 22 to turn OFF. The result is that thepulse output line 26 is pulled to the value of power supply voltage 25.Capacitor 21 may be a discrete part, or be the gate capacitances of theMOSFETs 22 and 23, or a combination of both.

It will be noted that when a steady sequence of alternating polarity lowvoltage edges 11 is applied, the two MOSFET switches 22 and 23 do notoperate in a true tri-state fashion; save for what might occur duringtransitions, one is always ON and the other is always OFF. Thisobservation leaves un-addressed, however, the issue of whether theMOSFET SWITCHES driven by the DIFFERENTIATING IMPULSE DRIVER operate inmake-before-break fashion or break-before-make fashion. Either may beobtained; what is shown operates as make-before-break. To obtainbreak-before-make it is merely necessary to interchange the two MOSFETswitches (note that they are a complementary pair, in that they havechannels of opposite polarities). An advantage of the make-before-breakarrangement is that is produces a lower ON resistance for the MOSFETs 22and 23.

Refer now to FIG. 3, which is a more detailed schematic 28 for a moresophisticated application of the basic technique set out in FIG. 2. Wehave kept the reference numerals the same in locations where the partsare in good correspondence, and have added new ones elsewhere. The basicoperation of the circuit 28 is the same, so we shall discuss only theincremental functionality provided by the additional components.

It will be noted that the input buffers for the DIFFERENTIATING IMPULSEDRIVER 29 have been made quite robust by operating several parts inparallel (12 a-c and 13 a-c). This is in keeping with the speeds beingreasonably high and the impedance to be driven being fairly low. Forexample, in one embodiment the range of pulse widths is 300 ns to 1,700ns, with rise times in the low nanoseconds and a 500 KHz repetitionrate.

The next topic of interest is the addition of the BI-STABLE SWITCHDRIVER 30. First, it is connected between ground and the junction node18 of the two transistors 16 and 17 of the IMPULSE DRIVER 29. Capacitor21 is now absent, save for what unavoidable strays and devicecapacitances (for transistors 22 and 23) remain. In fact, rather thanhave a capacitance there to act as a voltage storage mechanism, we nowprefer that there be as little capacitance from node 18 to ground aspossible. But something has to perform the “store-and-hold” functionthat allows transient impulses from the IMPULSE DRIVER 29 to cause asteady pulse output from the MOSFET SWITCHES 22 and 23. That somethingis the BI-STABLE SWITCH DRIVER 30, which is an active network builtaround amplifier 36. In addition, the voltage swing at node 18 has beenclamped through resistor 34 (a save-the-diodes current limit) by diode32 to a +7V CLAMP voltage, and by diode 33 to a −7V CLAMP voltage.

What the BI-STABLE SWITCH DRIVER 30 does is drive node 18 with apositive or negative voltage that is larger than the plus and minus 7VCLAMP voltages, respectively. The BI-STABLE SWITCH DRIVER is triggered,or switched, from one state or the other by an impulse of thecorresponding polarity from the IMPULSE DRIVER 29. It then drives node18 (i.e., one of the clamp diodes 32 and 33) through resistor 38, whichthen maintains a steady drive to the MOSFET SWITCHES 35 until the nexttransition in the LOW VOLTAGE EDGES 11.

Another way to think of the BI-STABLE SWITCH DRIVER 30 is as a voltagedriven current source. It “follows” the brief but powerful transientprovided by the IMPULSE DRIVER, and once perturbed has a source voltagehigher than the associated CLAMP voltage. The clamping action wins,however, and the clamped voltage substitutes for the now absenttransient impulse drive. But the voltage responsive current sourcebehavior remains, now in response to the clamping of its own output. Theaction is rather like a latch.

The reason for the 100 Ω resistor 34 is also now apparent: it is largeenough to keep the rather gutsy transient from the IMPULSE DRIVER fromfrying the diodes 32 and 33 and glitching the associated power supplies,yet small enough to not interfere with the clamping action.

The clamping action is important for error reduction, for the followingreason. If the drive to the MOSFET SWITCHES 35 was not clamped, then itwould in all likelihood exhibit droop or other non-constant behaviorbetween transitions. The amount of droop will be a function of how longit is allowed to occur, which is to say, it will be a function of pulseduration. This is bad, because when a transition does occur after therehas been some droop (or other drift), the further excursion in voltageneeded to reach the threshold for switching the MOSFETs will bedifferent from one instance to the next, according to the amount ofdroop. Since the slewing rates are finite, that translates into a pulsewidth dependent timing error in edge placement for the output pulse.Clamping fixes things so that any such effect of slewing rate/excursionto the threshold is constant for all pulses, which in essence, cancelsthe effect.

Another element that has been added to FIG. 3 is the AUXILIARY BI-STABLEOUTPUT DRIVER 31. It is another bi-stable voltage driven current source,built around amplifier 37 and coupled to a node (in this case line 26)through resistor 39. That is, when MOSFET 22 is ON, a positive voltage(and its current) is supplied by AUXILIARY BI-STABLE OUTPUT DRIVER 31,which is essentially an additional source in parallel with MOSFET 22. Asimilar negative voltage (and its current) is supplied when MOSFET 23 isON, instead. When one of the MOSFET SWITCHES is ON, (producing a known±7V) the associated output voltage would source or sink a particularcurrent through resistor 27 (its other end being a virtual groundprovided by the integrator). The component values chosen for theAUXILIARY BI-STABLE OUTPUT DRIVER 31 are such that IT supplies almostall of that current, leaving only a very little to be supplied by theMOSFET SWITCHES themselves. The reason for having the AUXILIARYBI-STABLE OUTPUT DRIVER 31 is as follows.

The MOSFETs 22 and 23 each have an ON resistance of about 10 Ω. It wouldnot be so bad if that value were constant, but it unfortunately variesas a function of temperature, and produces performance variations thatare detectable. By providing the extra drive from the AUXILIARYBI-STABLE OUTPUT DRIVER 31 (most of the current through the integratorload resistor 27 actually is due to sourcing and sinking by theAUXILIARY BI-STABLE OUTPUT DRIVER) those temperature induced variationsare swamped out. That is, the voltage drop across that temperaturevariable ON resistance is made very low, since almost no current flowsthrough it.

The circuit 28 shown in FIG. 3 is capable of precision operation. Theerror in edge placement for the output pulses, relative to the inputedges, can, with reasonable care, be made to be on the order of{fraction (2/10)} ppm. For the integrated output to have that kind ofaccuracy, appropriate care must be exercised with the various referenceand supply voltages, as well as with component selection generally. Andalthough there may be many combinations of passive component values andchoices for active devices, in the interest of complete disclosure wehave included in FIG. 3 actual component values and device specifiersfor an embodiment that meets the {fraction (2/10)} ppm performancespecification.

Lastly, we note that in some applications it may be desirable for thepulse generation circuits shown to operate in an unbalancedconfiguration, say, between ground and a thirty-six volt supply. Thismay be readily accomplished an appropriate shift in the various othervoltages. Also, it will be discovered by those who try it that thegeneration of pulses of much higher voltage is possible, too.

1. A method of producing pulses from a sequence of input signal edgesthat alternate in polarity, the method comprising the steps of: (a)differentiating the transitions of the input signal edges; (b)amplifying the differentiated transition of step (a); (c) generating inresponse to the amplified transitions of step (b) a current whosepolarity is determined by the polarity of the amplified transition towhich it is in response; (d) coupling the current of step (c) into anode; (e) clamping the voltage at the node of step (d) to positive andnegative values; (f) driving a load to a positive power supply voltagewith a switched ON first FET when the node of step (d) is clamped to afirst polarity; (e) driving the load to a negative power supply voltagewith a switched ON second FET when the node of step (d) is clamped to asecond polarity; and (f) one of the first and second FETs being anN-type FET and the other being a P-type FET.
 2. A method as in claim 1wherein the first and second FETs are in series and the polarities ofthe FETs are chosen to create make-before-break operation.
 3. A methodas in claim 1 wherein the first and second FETs are in series and thepolarities of the FETs are chosen to create break-before-make operation.4. A method as in claim 1 wherein the load is a resistor coupled to theinput of an integrator and further comprising the step of integratingcurrent pulses formed by the switching of the first and second FETs. 5.A method as in claim 1 further comprising the step of generating, inresponse to the voltage driving the load, a selected current ofcorresponding polarity that is coupled to the load and that supplies themajority of the current through the load.
 6. Pulse production apparatuscomprising: an impulse driver having an input for receiving a sequenceof input signal edges that alternate in polarity, that differentiatesthe input signal edges into positive and negative impulse transients,and that separately amplifies each such transient and presents theamplifications at a common output of the impulse driver; a voltageclamping circuit having an input and that limits positive and negativevoltages at that input to selected amounts; a voltage controlled currentsource having an input coupled to the common output of the impulsedriver and supplying current of a selected amount and polarity to theinput of the voltage clamping circuit; a complementary pair of FETswitches in series with each other, the complementary pair in parallelwith a supply voltage source, and the gates of the complementary pair ofFET switches each being connected to the input of the voltage clampingcircuit; and the junction of the pair of FET switches in series being apulse output coupled to supply an output pulse.
 7. Apparatus as in claim6 wherein the order of the complementary pair of FET switches as theyare in parallel with the supply voltage source producesmake-before-break operation.
 8. Apparatus as in claim 6 wherein theorder of the complementary pair of FET switches as they are in parallelwith the supply voltage source produces break-before-make operation. 9.Apparatus as in claim 6 further comprising a integrator having an inputand also further comprising an integrator input resistor coupled betweenthe pulse output and the input of the integrator.
 10. Apparatus as inclaim 6 wherein the peak-to-peak voltage of the output pulse is greaterthan the peak-to-peak voltage of the input signal edges that alternatein polarity.
 11. Apparatus as in claim 6 further comprising a loadcoupled to the pulse output and also further comprising another voltagecontrolled current source, having an input coupled to the pulse output,that generates a current of selected size and polarity that is coupledto the load, and that supplies the majority of the current for the load.